Conventionally, water absorbent resin has been widely used to constitute sanitary materials such as a disposable diaper, a sanitary napkin, an incontinence pad, and the like, so as to absorb aqueous liquid such as body fluid and the like. Further, water absorbent resin has been used also to constitute water absorbing articles for horticulture, agriculture, and the like.
Further, it is known that, in view of a handling property, safety, and hygiene, particles of the water absorbent resin powder are granulated (bound) by using water, aqueous liquid, water-soluble polymer, or the like so as to have a desired particle diameter so that the resultant is used as a water absorbing agent or the like. Further, it is known that: water, aqueous liquid, and the like are added to a deposition of the water absorbent resin powder which deposition has appropriate thickness so as to obtain a sheet-shaped water absorbent resin layer, and then the resultant is granulated so as to have a desired particle diameter, thereby producing a water absorbing agent or the like.
However, according to the conventional granulation method, also after the granulation, a large amount of ungranulated water absorbent resin powder (particularly, powder whose particle diameter is less than 150 μm) remains. Thus, in case of using the resultant water absorbing agent and the like in a disposable diaper and the like, its liquid permeability drops. Further, in case of improving properties such as absorbency against pressure or a similar property by further surface crosslinking the granulated water absorbing agent or the like, the ungranulated powder preferentially absorbs the surface crosslinking agent, so that it is impossible to evenly disperse the surface crosslinking agent over whole the water absorbing agent or the like. As a result, it is difficult to obtain the water absorbing agent or the like which satisfies the desired property (quality) level.
Further, in case of granulating the water absorbent resin powder by adding water, aqueous liquid, or the like, it is necessary to add a large amount of water, aqueous liquid, water-soluble polymer, or the like so as to realize sufficient granulation, so that the energy cost increases at the subsequent drying step. In case of granulating the water absorbent resin powder by adding various kinds of binder such as water-soluble polymer or the like, the binder itself causes the liquid permeability, absorbent property, and the like of the resultant water absorbing agent to drop.
As a method for solving the problems of the conventional granulation method, Patent Document 1 describes a production method of particulate water absorbent resin. According to the method, when supplying water to water absorbent resin powder so as to carry out granulation, it is possible to reduce an amount of ungranulated particles after the granulation and it is possible to reduce also the drying cost after the granulation by suppressing an amount of supplied water required in the granulation. Specifically, Patent Document 1 describes “a method for producing particulate water absorbent resin comprises the steps of: supplying moisture to water absorbent resin powder so as to bind particles of the powder; and obtaining granulated water absorbent resin particles from the bound particles of the powder, wherein the supplied moisture is in a form of moisture vapor”.
Further, Patent Document 2 describes a method in which: water absorbent resins are made gelatinous with a large amount of warm water so as to cause the water absorbent resins to adhere to each other, and evaporation energy of the large amount of water is used to dry the gelatinous water absorbent resins. Specifically, Patent Document 2 describes “a method for producing a granulated water absorbent resin by mixing water absorbent resin powder and aqueous liquid comprises the steps of: heating the aqueous liquid which has not been mixed; and mixing the heated water and the water absorbent resin powder at a high speed”.    [Patent Document 1]            Japanese Unexamined Patent Publication Tokukai 2005-54151 (Publication date: Mar. 3, 2005)            [Patent Document 2]            Japanese Unexamined Patent Publication Tokukaihei 11-106514 (Publication date: Apr. 20, 1999)        